In the past, as the method of attaching a device inside of a pipe to uniformly mix fluid flowing through the inside of the pipe, as shown in FIG. 12, use of a swirl blade type static mixer element 81 has been the general practice (see, for example, Japanese Patent Publication (A) No. 2001-205062). Usually, the static mixer element 81 is comprised of a square plate twisted 180 degrees about its longitudinal axis as a minimum unit member and has a plurality of such minimum unit members integrally connected in series so that the twisting directions become mutually different directions. This static mixer element 81 is arranged in a pipe 82, male connectors 83 are attached to the two end parts of the pipe 82, flare nuts 85 are attached, and fastening nuts 84 are fastened, whereby a static mixer is formed. At this time, the outside diameter of the static mixer element 81 is designed to be substantially equal to the inside diameter of the pipe 82, so the fluid is able to be effectively agitated.
However, the method of mixing fluid using this conventional static mixer is to agitate flowing fluid along the flow, so as shown in FIG. 13(a), it is possible to make the distribution of concentration in the diametrical direction of the pipe uniform without any unevenness, but as shown in FIG. 13(b), it is not possible to make the distribution of concentration in the axial direction (flow direction) uniform without any unevenness. For this reason, for example, when mixing water and a chemical at the upstream side of the static mixer, if the mixing ratio of the chemical temporarily increases, the fluid will pass through the static mixer in a state partially denser in concentration in the flow path. At this time, even if the water and chemical are stirred while made uniform in concentration in the diametrical direction, in the axial direction (flow direction), locations in the flow path where the concentration partially becomes denser will end up flowing to the downstream side in the dense state as they are without being diluted much at all (see FIG. 13(b)). Due to this, when connected to a semiconductor washing apparatus, in particular, an apparatus which directly coats the surface of a semiconductor wafer with a chemical to perform various types of treatment, there was the problem that different concentrations of the chemical were coated on the surface of the semiconductor wafer and thereby caused defects.
As a method for avoiding unevenness in the distribution of concentration in the axial direction (flow direction), the method of installing a tank in the middle of the flow path, storing the fluid temporarily in the tank, making the concentration in the tank uniform, then running the fluid (not shown) etc. may be mentioned. However, there were the problem that a large space was required for installing the tank and therefore the apparatus became larger, the problem that transport of the fluid from the tank again required a pump, piping, etc., so the number of the parts used increased, and the problem that cost was incurred for installing the pipeline. Further, with this method, the fluid stagnates in the tank. If the fluid stagnates, it becomes a cause of proliferation of bacteria, the bacteria proliferating in the tank flows into the pipeline, and, in a semiconductor production line, deposit on the semiconductor wafer and cause defects.
As another method for avoiding the unevenness in the distribution of concentration in the axial direction (flow direction), as shown in FIG. 14, there was a branching and diluting apparatus for branching flow paths and diluting the fluid (for example, see Japanese Patent Publication (A) No. 8-146008). This apparatus analyzed a sample solution flowing through a tube 91 at a constant speed. It provided a branching part 92 branching the flowing sample to a plurality of flow paths in the middle of the flow path so as to divide the sample solution, changed the inside diameters or lengths of the tubes 93 and 94 of the branch flow paths, combined the flows again at a merging part 96 before a detector 95, and utilized the time difference at which the sample solution was detected for dilution.